1. Field of the Invention
The present invention relates to an electronic tachometer and to a brushless dc motor system having combined commutation control, tachometer and position encoding functions.
2. Description of the Prior Art
In most velocity-responsive servo systems, the velocity of a mechanical element is sensed by a tachometer, the output of which is used to control the speed of a motor driving the element. A position encoder also may be used to ascertain the position of the mechanical element. Typically, such a system employs a dc motor, a separate tachometer and a separate position encoder. If a brushless motor is used, additional commutation circuitry must be used which is responsive to the angular position of the rotor.
One objective of the present invention is to provide a system to which the tachometer, position encoding and brushless dc motor communtation control functions are combined, so that independent electromechanical devices are not required to perform these functions separately. More generally, another objective is to provide an electronic tachometer for producing a signal indicative of the velocity of a mechanical element. The same transducer used by the tachometer also may be used for brushless motor commutation control and/or position encoding, and it is a further objective of the present invention to provide means for such multipurpose transducer utilization.
An example of a servo system employing these components is a medium speed printer of the type which may be used as the output device for a minicomputer. In such a printer, a paper drive mechanism is used to feed paper from a roll or zig-zag folded stack past a printing station. Typically, the paper feed motion is not continuous. Rather, the paper is held stationary while an entire line of type is printed. Then the paper quickly is advanced by one line, and stopped again for printing of the next row. A speed of 1500 lines per minute is typical for such a printer, with either six or eight lines per inch. Operating at this speed, 40 msec are available to print the entire row and to advance the paper to the next line. Obviously, the paper feed operation must be accomplished very rapidly. However, the acceleration must not be so great as to tear the paper. Moreover, the distance advanced for each line must be uniform so that the resultant document has a fixed spacing between each line.
These paper feed requirements in the past have been met by using a drive motor in a servo system employing a separate tachometer and position encoder. A velocity profile is established in which the paper initially is accelerated at a rate which will not cause tearing. When the paper moves to within a fixed distance from the next print line, as established by the output of the position encoder, deceleration begins, again with a fixed velocity profile. Throughout the operation, the motor speed is controlled by comparing the actual velocity from the tachometer with the desired velocity, the latter being indicated by a control signal which is a function of the distance travelled by the paper.
An object of the present invention is to provide an improved drive motor servo system and components therefor, which may be used advantageously in a paper feed mechanism for a printer.
Various features are desirable in the tachometer portion of such a servo system. Advantageously, the tachometer output signal should be linearly related to speed, and preferably should have a polarity which indictes the direction of motion of the mechanical element, the speed of which is being measured. Preferably, any ripples superimposed on the tachometer output signal should be of a high enough frequency to permit filtering without reducing the system bandwidth. The ripple amplitude should be independent of the output signal amplitude. There should be no motor-tachometer electromagnetic coupling and the motor-tachometer torsional resonant frequency should be as high as possible. A further object of the present invention is to provide an electronic tachometer having all of these features.
Advantageously, an electronic tachometer may employ an optical or other absolute encoder which directly indicates the motion of the mechanical element being measured. An objective of the present invention is to utilize such an encoder or transducer both as a component of an electronic tachometer and to yield commutation information for properly switching the windings of a brushless motor utilized in the system. Position encoding also may be obtained from the same transducer.
One approach utilized in the prior art to obtain both velocity and position information involves the use of an "Inductosyn". This consists of a pair of printed circuit boards each containing a comb-like pattern. The boards are mounted respectively on the moving and stationary elements of the device being measured, in transformer relationship with one another. A high frequency ac signal is fed into one of the comb-like windings, and the signal which is inductively coupled to the other winding is sensed by appropriate circuitry. As the two comb-like windings are moved relative to one another, the induced signal is demodulated and appropriately processed to obtain velocity and position information. An objective of the present invention is to provide means for obtaining velocity and position information without the requirement of using relatively movable, inductively coupled transducer components.
Another shortcoming of the prior art which is overcome by the present invention concerns commutation in a brushless dc motor. In the past, switching has been controlled by transducers using mechanical interruption of an optical beam, or by Hall-effect devices that are switched by passage of the permanent magnet rotor poles or by a separate magnetic encoder. An objective of the present invention is to provide commutation control utilizing a simplified optical transducer that may also be used in conjunction with an electronic tachometer and/or a position encoder.